The present invention relates to radio frequency antennas and more particularly, to loop antennas which generate a rotating field.
In certain types of electronic systems it is known to provide one or more loop antennas wherein coupling between an antenna and its proximate surrounding is high, but wherein the design of the antenna is such that coupling between the antenna and its distant surrounding (i.e., about one wavelength or more distant from the antenna) is minimized. Such antennas are generally used for near-field communications or sensing applications where the term "near field" means within one half wavelength of the antenna. Examples of such applications include communications with implanted medical devices, short range wireless local area communications networks for computers and radio frequency identification systems including electronic article surveillance (EAS) systems. Generally, the coupling to these loop antennas is primarily via magnetic induction.
For example, radio frequency identification (RFID) systems usually include both a transmit antenna and a receive antenna which collectively establish a detection zone, and tags which are attached to articles being protected. The transmit antenna generates an electromagnetic field which may be fixed or variable within a small range of a first predetermined frequency. The tags each include a resonant circuit having a predetermined resonant frequency generally equal to the first frequency. When one of the tags is present in the detection zone, the field generated by the transmit antenna induces a voltage in the resonant circuit in the tag, which causes the resonant circuit to resonate and thereby generate an electromagnetic field, causing a disturbance in the field within the detection zone. The receive antenna detects the electromagnetic field disturbance, which may translate to item identification data related to the protected article attached to the tag in the detection zone. Special antenna configurations have been designed for such purposes.
One conventional antenna has a two loop, figure eight configuration. In such a two loop antenna, a weak detection field or "hole" occurs at the center of the detection zone, which is the zone generally parallel to the crossover of the loops of the figure eight. The hole is especially prominent when the tag is oriented in a position that is normal or perpendicular to the axis of the crossbar.
A three loop antenna is commonly used to address the issue of weak field production in the center zone. However, a three loop antenna which is large enough to cover a volume of several cubic meters will have a self-resonance below 13.56 MHz, which is a desired frequency for certain tag applications. Accordingly, such an antenna cannot be tuned to 13.56 MHz.
One conventional technique for developing the field in the center zone is by simply driving a center loop with the same current source as the primary loop. However, this technique is not optimum, since "hot" and "cold" areas develop from positive reinforcement and destructive cancellation, respectively, due to field components of the figure eight and center loop with opposite polarity. By rotating the field, the antenna basically averages the hot and cold spots, and provides uniform field production.
Another conventional technique for generating a rotating field is to drive the center loop 90 degrees out of phase with respect to the other loops using a series/parallel matching network.
Both of these conventional schemes for providing a rotating, uniform field require that the center loop be electrically connected to the figure eight loop. One conventional connection scheme is to electrically connect the center loop to the figure eight loop through a phase shifting network. The phase shifting network adds cost and complexity to the antenna. Also, losses in the network components reduce the efficiency of the antenna.
Accordingly, there is a need for a rotating field antenna which does not require such an electrical connection and which is well-suited for radio frequencies in the range of 13.56 MHz. The present invention fulfills these needs.